While Siri is not the sole figure in the classical tragedy that ends with Google’s disruption—a play whose large cast includes heel-nippers like Blekko, vertically-targeted engines like Kayak, richly populated databases like Yelp and pretty much everyone else in consumer software—it is hard to read Schmidt’s letter without a sense that he knows that Apple and Siri are Google’s key antagonists.

Apple’s newest plaything appears about 15 times in Schmidt’s letter. Siri, Schmidt writes, is “an entirely new approach to search technology,” one that represents a “significant development.” He even points to MG’s recent piece about Siri’s potential to make his case. (For more on how this impacts search, see my post about Siri + Quora).

Schmidt’s argument—a highly cogent and persuasive piece of propaganda—is dead-on: In the narrative arc of Google’s story, Siri is indeed a significant development. In fact, Siri is arguably the most significant development in Google’s story since, well, the advent of Google.

Sure, this story includes Facebook, the social web juggernaut that Google failed to see charging towards it on the distant horizon, and the Bing/Yahoo nexus that Microsoft assembled at magnificent cost. But in the retrospective view of of 3-5 years from now, these developments—while major—will likely pale in comparison to that fateful April day when Apple acquired Siri.

This is because Siri transforms computers from “passive” participants in the search process to “active” ones and in so doing urinates all over Google’s model. Instead of taking queries and then passively spitting out 10 blue links—which you then have to mine for the correct information—Siri actively goes and gets the correct information, herself. If you don’t give Siri the input she needs, she’ll ask you directed questions until you do. If the goal of your search is a specific action (like buying a movie ticket, reserving a table or calling a taxi), Siri can skip all the steps Googling would require and just do it. At least that is the idea. Siri is still quite limited in what it can do, but the writing is on the wall.

When a technology comes along that eliminates the need to follow previously-required processes, we call it disruptive. There’s no other way to slice it.

Many classical tragedies have ironic twists at their center, and this one is no different: When they launched Android against the iPhone, Google and Schmidt attacked Apple’s homeland and turned a longtime friend (Steve Jobs) into a mortal enemy. And now, with Siri, Jobs will have his posthumous revenge. Because make no bones about it: despite Jobs’ original claims that the Siri acquisition was not about search, Siri and the ecosystem of Siri-optimized APIs sure to evolve in its wake represent a bigger mortal threat to Google’s search business than anything Google has ever faced.

So Microsoft, you can can stop bankrolling the anti-Google lobby in Washington. Feds on the subcommittee, you can put away your knives: the weak have little to fear from Goliath.

In technology, at least, someone always comes along with a rock and a slingshot.